Turbo machinery, for example turbine engines, generally comprises a compression system. The compressor is powered by a turbine, with the compressor and turbine forming what may be referred to as a spool. Commonly in gas turbines the compressor is exclusively axial flow in nature. The disadvantage of an all axial compressor is that at small engine sizes the final stages of the compressor become very small which results in large aerodynamic losses and poor compressor efficiency.
It is known to combine different compressor configurations in turbo machinery designs. In U.S. Pat. No. 4,678,398 (Dodge et al.) there is described mixed flow compressor upstream of an axial flow compressor. While resistant foreign object damage, this design does not address the issue of poor efficiency of the downstream axial compressor if the final stages of that compressor are physically small. In U.S. Pat. No. 6,488,469 (Youssef et al) there is described a mixed flow compressor upstream of a circumferential compressor. Whilst aerothermally efficient at small engine sizes, this latter design will result in relatively large engine diameter due to the size requirements of the circumferential compressor.
Hence a compression system which has a relatively high ratio of efficiency to compressor diameter, which avoids the need for small rotor blade size, with no penalty on operational envelope, is highly desirable.